1. Field of the Invention
The present invention relates to a differential operational amplifier and a bandgap reference voltage generating circuit applying the differential operational amplifier, and particularly relates to a differential operational amplifier that can reduce the temperature for the input voltage and a bandgap reference voltage generating circuit applying the differential operational amplifier.
2. Description of the Prior Art
In the field of circuit design, a reference voltage generating circuit is always applied to generate an accurate reference voltage as a voltage standard for other devices. Voltage generating circuits can be classified to various kinds, and one of them is a bandgap reference voltage generating circuit. The devices inside such circuit adjusts the voltage or the current thereof responding to a temperature coefficient, such that the generated reference voltage can be kept at a stable value.
The bandgap reference voltage generating circuit always comprises a differential operational amplifier to control the operation for the bandgap reference voltage generating circuit via a first voltage and a second voltage. The first voltage and the second voltage change corresponding to a temperature difference. The differential operational amplifier always comprise the NMOSFET input pair or the PMOSFET input pair illustrated in FIG. 1 to receive above-mentioned first voltage and second voltage. Either the first voltage V1 or the second voltage V2 has a negative correlation with a temperature. Therefore, if the differential operational amplifier operates at a lower operational voltage VDD and operates at a low temperature, the voltage difference VDS between a source terminal and a drain terminal for the PMOSFET Pa of the PMOSFET input pair is suppressed (as shown in FIG. 2), such that the PMOSFET Pa operates in a linear region thus the differential operational amplifier comprises a smaller amplifier gain. On the opposite, if the differential operational amplifier operates at a high temperature, the voltage difference VDS between a source terminal and a drain terminal for the NMOSFET Na of the NMOSFET input pair is suppressed, such that the NMOSFET Na operates in a linear region thus the differential operational amplifier comprises a smaller amplifier gain.
In the above-mentioned case, the VGS (the voltage difference between the gate terminal and the source terminal) must be increased, if the PMOSFET Pa or the NMOSFET Na need constant output currents. However, the difference between the first voltage V1 and the second voltage V2 change (as shown in FIG. 3) if the VGS increases, thereby the reference voltage generated by the reference voltage generating circuit is also affected.